go/types: simplify under() and fix a crash

This is a port of CL 363665 from types2 to go/types.

Change-Id: I20c4e20ab97f1e4de66a29095dc4a9b160810fe5
Reviewed-on: https://go-review.googlesource.com/c/go/+/364897
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/lookup.go

@@ -469,6 +469,13 @@ func (check *Checker) assertableTo(V *Interface, T Type) (method, wrongType *Fun
 // Otherwise it returns (typ, false).
 func deref(typ Type) (Type, bool) {
 	if p, _ := typ.(*Pointer); p != nil {
+		// p.base should never be nil, but be conservative
+		if p.base == nil {
+			if debug {
+				panic("pointer with nil base type (possibly due to an invalid cyclic declaration)")
+			}
+			return Typ[Invalid], true
+		}
 		return p.base, true
 	}
 	return typ, false

src/go/types/testdata/check/cycles.src

@@ -45,6 +45,7 @@ type (
 
 	// pointers
 	P0 *P0
+	PP *struct{ PP.f /* ERROR no field or method f */ }
 
 	// functions
 	F0 func(F0)

src/go/types/type.go

@@ -21,13 +21,10 @@ type Type interface {
 // under must only be called when a type is known
 // to be fully set up.
 func under(t Type) Type {
-	switch t := t.(type) {
-	case *Named:
+	if t, _ := t.(*Named); t != nil {
 		return t.under()
-	case *TypeParam:
-		return t.iface()
 	}
-	return t
+	return t.Underlying()
 }
 
 // If x and y are identical, match returns x.

src/go/types/typexpr.go

@@ -308,6 +308,7 @@ func (check *Checker) typInternal(e0 ast.Expr, def *Named) (T Type) {
 
 	case *ast.StarExpr:
 		typ := new(Pointer)
+		typ.base = Typ[Invalid] // avoid nil base in invalid recursive type declaration
 		def.setUnderlying(typ)
 		typ.base = check.varType(e.X)
 		return typ